Image-guided targeting has a growing role in a range of medical procedures. At its foundation, image-guidance involves the computer correlation of near real time imagery with an historic three-dimensional (3D) volumetric data to determine the spatial location of patient anatomy. That is, image-guided targeting includes techniques and processes that are used to create images of the human body. Typical examples of 3D volumetric data for image-guided targeting include computed tomography (CT) or magnetic resonance imaging (MRI).
The use of this image-guided targeting has been best described for precise localization of patient bony anatomy in 3D space using projection x-rays or cone beam CT. The tacit assumption that underlies most image-guidance is that skeletal anatomy can provide a reliable reference for nearby soft tissue. Nevertheless, sub-millimetric targeting accuracy is possible with such techniques, thereby enabling even the most precise surgical procedures to be performed under image-guidance.
The primary role of x-rays in image-guidance is to define the 3D location of bony anatomy for image-correlation. Their relative capacity to penetrate skin is the cardinal feature of x-rays that enables them to be used for imaging. In contrast, the kilovoltage energy x-rays used in image-guidance are characterized by a much greater proclivity to be scattered off bone, and therefore a much greater likelihood of being blocked from transmission through the tissue being imaged.
Although imaging with x-rays is robust, the challenge of using ionizing radiation burdens this approach because ionizing x-rays are potentially harmful to patients and the medical team. As such, current technologies using ionizing x-rays for image-guidance is rarely done on a continuous basis. For example cone beam CT scans are generally only produced at the start of a several minute to several hour procedure, while projection x-rays used for image correlation are only generated every 20 to 60 sec. The infrequency of such “real time” imaging means that instantaneous patient movement goes undetected, and will result in therapeutic inaccuracies.
Further, in radiosurgery and radiation therapy, tumors are destroyed with a beam of radiation. For instance, methods have been developed in which a mechanical gantry is used to move the beam source. Two x-ray imaging cameras are used to compute the position of the patient's skull during treatment. This x-ray imaging is repeated several times per minute.
However, x-ray imaging in this context has several limitations. Firstly, as previously discussed, x-ray imaging requires ionizing radiation, and is thus invasive for the patient and for the operating team. Radio-surgical procedures may last for up to one hour. Taking x-rays continuously during the entire treatment would expose the patient to a substantial amount of radiation dose from x-ray imaging alone. Secondly, in x-ray imaging it is necessary that the x-ray source be on one side of the patient, and the x-ray detector be one the other side of the patient. Thus sufficient space for placing separate source and detector units is necessary.
What is needed is a truly real time imaging modality that can be correlated to 3D patient anatomy.